Titanium and titanium alloys are of prime interest for aerospace applications because of their favorable strength to density and high temperature characteristics compared to other aerospace materials. Titanium has been shown to be subject to embrittlement by oxidation and oxygen contamination during high temperature exposure. Because of the loss in ductility of titanium alloys exposed to oxygen containing environments at high temperatures, their use has been restricted to applications where the sustained environmental temperature is below 580.degree. K. or where the short term temperature spiking is limited to peak temperatures below 800.degree. K. These limitations on high temperature applications of titanium prevent the realization of the maximum mechanical and structural efficiency of the material.
The above limitations are particularly restrictive in high temperature aerospace application such as our nation's Space Shuttle program and in research and development of advanced space transport concepts. The design of an effective thermal protective system has been particularly troublesome for the Space Shuttle program. The current thermal protection system (TPS) in use on the Shuttle program, under the direction of the National Aeronautics and Space Administration, utilizes ceramic tiles to shield high temperature structural panels of the orbitor on re-entry. These tiles are fragile and subject to cracking, as well as exhibiting problems in adhesion.
Since every 0.4536 kg (pound) saved in the thermal protection system (TPS) is a 0.4536 kg (pound) of available additional payload to orbit, it is essential to have a TPS that has a low mass and is capable of enduring the service conditions for 100 missions. Research has experimented with various methods of increasing the efficiency of TPS. One approach is to increase the thermal operating temperatures of an existing material through use of some coating scheme. Titanium has been the material most often thought to have the highest potential, due to its inherent resistance to thermal stress and high strength to weight ratio.
The prior art is replete with coating methods that attempt to shield the substrate titanium from the effects of oxidation and embrittlement which result from high temperature exposure in oxygen environments. These prior art coating methods range from epitaxy chemical vapor disposition systems (CVD) to special processes where flaked aluminum is baked on the substrate metal. A limitation to these prior art systems for coating titanium is that they fail to elevate the operating temperature enough to permit use as a thermal protection system on a reusable space orbitor, or produce coatings that are bulky and take on physical or mechanical properties separate from the substrate metal. The ability to apply a submicron coating to foil gage titanium is required for development of improved multiwall panels to replace the ceramic tiles currently in use on space orbitors, without increasing weight. It is also important that the coating materials are thin enough to diffusion bond with the substrate metal without the coating itself having separate mechanical and physical properties which result in mechanical spallation or chipping and cracking when the finished product is flexed physically and/or thermally.
Current art efforts to resolve these problems have generally failed to overcome these restrictive limitations on the use of titanium in elevated thermal environments. An example of current art efforts include special processes employing expensive heat treatment steps, where various chemicals are applied to titanium including flaked aluminum, then the materials are baked in a kiln to heat diffusion bond the coating to the substrate metal. While this process does impart protection from oxygen diffusion through to the substrate metal, the coatings exhibit negative features that preclude the materials use in space applications. For instance, prior art techniques result in a coating at least 25 microns in thickness while the present invention results in coatings about 1 micron thick. This additional mass results in a coating that has its own mechanical properties leading to cracking and/or spallation, while the oxygen barrier coatings of the present invention are sufficiently thin to react with the substrate titanium to a degree that their separate mechanical properties are subsumed by those of the substrate. The extra thicknesses of the state of the art processes translate to additional mass and abrogates to a degree the advantages of titanium high strength to weight ratio, which led to titanium selection as a substrate material in TPS applications.
The present invention reveals a process for coatings that result in thermal protection virtually without adding weight to the base metal. The test results herein described show a total weight increase as small as 3.75 micrograms per square centimeter which has no impact on the weight of the structures employing the protection. Most prior art adds two to five milligrams of mass per square centimeter of protection, thus reducing the attractiveness of these coatings for in orbital thermal protection systems. It was the need to find a process that can immunize foil gage titanium for use in space thermal protection systems without adding appreciable mass or introducing the additional mechanical limitations that led to the present invention. These benefits are engendered with proven inexpensive techniques, namely, electron-beam vapor deposition and sputtering.
Accordingly, it is an object of the present invention to provide a means of protecting titanium from oxygen in high temperature environments up to 1300 degrees Fahrenheit.
Another object of the present invention is to provide a coating procedure comprising electron-beam deposition followed by sputtering.
A further object of the present invention is to provide a coating for titanium which interacts with the base alloy to form a protective layer of aluminides and silicides.
Yet another object of the present invention is to provide a means of coating titanium to form a layer functioning as a diffusion barrier coating.
Another object of the present invention is to provide a means of protecting titanium from oxygen in high temperature environments that is inexpensive to apply and employs techniques proven efficacious in the art field.
Another object of the present invention is to provide a coating for titanium that blocks oxygen diffusion to the alloy thereby eliminating the source of alloy embrittlement.
Another object of the present invention is provide a coating for titanium that does not appreciably add mass to the base material.
Yet another object of the present invention is to provide a coating for titanium that allows the alloy to function in high temperature environments such as Space Shuttle re-entry without the alloy losing tensile elongation.
A further object of the present invention is to provide a coating for titanium that is submicron in depth.
Another object of the present invention is to provide a coating for titanium that can be applied to foil gage titanium including those less than 0.001 inch in thickness.
Yet another object of the present invention is to provide a coating process that results in a coating virtually free of cracking and spallations.